Limit level switches are manufactured and sold by Endress+Hauser. These involve, for example, so-called oscillation forks, which are excited to oscillate at their resonance frequency. If these sensors interact with the medium, then the frequency and also the amplitude of the oscillations change. In the case of bulk goods, the amplitude is most often considered, while, in the case of liquids, the frequency is more likely to be evaluated. If the fork oscillates freely, then the oscillation frequency is higher than in the condition where the medium covers the fork. Thus, it can be inferred from a reduction of the resonance frequency that a covering of the fork by the medium has occurred, and consequently, that a fill level has been reached by the medium. The contrary applies to the case where a fill level is subceeded, or fallen beneath. Depending on the case, an appropriate switching signal is issued. The limit value itself is dependent on, among other things, the dimensioning of the sensor unit and its installation position.
An output signal is normally produced by such a measuring arrangement, which signal carries the information concerning whether the fill level has been subceeded or exceeded. Thus, from the measurement signal, which carries information concerning the amplitude and the resonance frequency of the sensor, a single piece of information is distilled; that is, it amounts to an information reduction, but with such there is also an information loss. This output signal, indeed, does permit the control of actuators such as pumps, valves, etc., but information, such as e.g. the gradual change of resonance frequency with the amount of covering, can no longer be exploited.